High-strength bolt superior in delayed fracture and resistance and relaxation resistance

ABSTRACT

Disclosed is a high-strength bolt having a tensile strength of 1,200 N/mm 2  or more and superior in delayed fracture resistance and relaxation resistance, prepared by wire-drawing a bolt steel containing the following elements: C: 0.5 to 1.0% (mass %, the same shall apply hereinafter), Si: 0.55 to 3%, Mn: 0.2 to 2%, P: 0.03% or less (but not 0%), S: 0.03% or less (but not 0%), and Al: 0.3% or less (but not 0%), and containing proeutectoid ferrite, proeutectoid cementite, bainite and martensite at a total areal rate of less than 20% and pearlite in balance; cold-heading the wire into a bolt shape; and then bluing the bolt in a temperature range of 100 to 500° C.

TECHNICAL FIELD

The present invention relates to a high-strength bolt mainly for use inautomobiles, and in particular to a high-strength bolt having a tensilestrength (strength) of 1,200 N/mm² or more that is superior in delayedfracture resistance and relaxation resistance.

BACKGROUND ART

Carbon alloy steels (such as SCM435, SCM440, and SCr440) are used forcommon high-strength bolts, and these bolts are given a desirablestrength by quenching and tempering. However, common high-strength boltsused in automobiles and various industrial machines have a risk ofgenerating delayed fracture when they have a tensile strength in therange above approximately 1,200 N/mm², and thus, have restriction inuse.

The delayed fracture includes both phenomena occurring in corrosive andnoncorrosive environments, and the reasons for its occurrence are saidto be complicated by various factors, and thus, it is difficult tospecify the reasons indiscriminately. The regulatory factors responsiblefor the delayed fracture include tempering temperature, structure,material hardness, grain size, various alloy elements, and others, butthere is currently no effective means to prevent the delayed fracture,and various methods are studied by trial and error even now.

There are proposed many methods of preventing the delayed fracture(e.g., Patent Documents 1 to 3). In these methods, the delayed fractureof high-strength bolts having a tensile strength of 1,400 N/mm² or moreis reduced by adjusting the contents of various major alloy elements,but the risk of the delayed fracture is not completely eliminated, andthe application thereof still remains in extremely limited areas.

Patent Document 4 discloses a method of improving the delayed fractureresistance further. In Patent Document 4, a high-strength bolt having atensile strength of 1,200 N/mm² or more is produced by preparing ahigh-strength bolt steel not in a quenched and tempered structure but ina pearlite structure and then by strongly wire-drawing the bolt steel.The pearlite structure introduced in the high-strength bolt has anaction to trap hydrogen at the interface between cementite and ferriteand to reduce hydrogen accumulated at the interface, and thus, improvesthe delayed fracture resistance.

However, the pearlite steel has a problem of its own. That is,tightening bolts used at high temperature occasionally show a phenomenonof deterioration in proof stress ratio and thus in tightening forceduring use, and such a phenomenon is called relaxation (relaxation ofstress). There is a concern about decline in properties for thephenomenon (e.g., in relaxation property), especially when a pearlitesteel instead of a quenched and tempered steel is used in production ofbolt. Such a phenomenon may lead to elongation of the bolt and possibledeterioration in its initial tightening force, and thus, bolts used, forexample, around automobile engine should also be superior in relaxationresistance. However, the relaxation property is not considered well inconventional high-strength bolts, except the bolt described in PatentDocument 5.

In Patent Document 5, a pearlite steel having a particular compositionis wire-drawn strongly and cold-headed into the bolt shape, and the boltis subject to a bluing treatment in a temperature range of 100 to 400°C. The bluing prevents plastic deformation by the age hardening with Cand N, improves the strength and the proof stress ratio of the bolt, andalso prevents thermal settling at a temperature of 100 to 200° C., andthus, improves the relaxation resistance. Although Patent Document 5 isan invention aimed at improving relaxation resistance, it does notdisclose the relationship between Si content and the relaxationresistance and claims that the Si content should be 0.5% or less becausean excessive Si content leads to deterioration of the ductility of thesteel material after wire-drawing as well as significant deteriorationof the cold-heading efficiency.

-   Patent Document 1: Japanese Unexamined Patent Publication No.    60-114551-   Patent Document 2: Japanese Unexamined Patent Publication No.    2-267243-   Patent Document 3: Japanese Unexamined Patent Publication No.    3-243745-   Patent Document 4: Japanese Unexamined Patent Publication No.    2000-337332-   Patent Document 5: Japanese Unexamined Patent Publication No.    2001-348618

SUMMARY OF THE INVENTION

An object of the present invention, which was made under thecircumstance above, is to establish a method to further increase therelaxation resistance of a high-strength bolt in a pearlite structurehaving a tensile strength of 1,200 N/mm² or more that is superior indelayed fracture resistance.

The high-strength bolt according to the invention has a tensile strengthof 1,200 N/mm² or more that is superior in delayed fracture resistanceand relaxation resistance, characterized by being prepared by:wire-drawing a bolt steel containing the following elements: C: 0.5 to1.0% (mass %, the same shall apply hereinafter), Si: 0.55 to 3%, Mn: 0.2to 2%, P: 0.03% or less (but not 0%), S: 0.03% or less (but not 0%), andAl: 0.3% or less (but not 0%), having at a total areal rate ofproeutectoid ferrite, proeutectoid cementite, bainite and martensite ofless than 20%, and containing pearlite in balance; cold-heading the wireinto a bolt shape; and then bluing the bolt in a temperature range of100 to 500° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 includes schematic explanatory views illustrating a stud boltused in a delayed fracture resistance test.

FIG. 2 is a photograph replacing drawing, showing the bainite structure.

FIG. 3 is a photograph replacing drawing, showing the proeutectoidcementite structure.

FIG. 4 is a graph showing the relationship among the Si content,presence or absence of bluing, and the relaxation value.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

After intensive studies to solve the problems above, the inventors havecompleted the present invention by finding that, although it was notpossible to improve the relaxation resistance only by bluing a boltsteel or only by increasing the Si content to a certain value or morewithout bluing, it was possible to improve the relaxation resistancesignificantly by combining an addition of Si in an amount more than aparticular value with bluing treatment.

The bolt steel for use in the present invention (high-strength boltsteel) has generally a wire or rod shape; more specifically, it includesboth a steel material (wire rod) hot-worked into the wire or rod shapeand then heat-treated and a steel material (steel wire) obtained bycold-working the wire rod, for example, mainly by wire drawing; and thesteel wire is preferable. The high-strength bolt steel is a kind ofpearlite steel, more specifically, a steel having at a total areal rateof proeutectoid ferrite, proeutectoid cementite, bainite and martensiteof less than 20% and containing pearlite in balance (i.e., the arealrate of the pearlite structure is more than 80%). Increase in theamounts of proeutectoid ferrite and proeutectoid cementite makes strongwire-drawing difficult because of breakage of wire during wire-drawing,and makes it difficult to adjust the strength of the bolt to apredetermined value or more. Increase in the amounts of proeutectoidcementite and martensite leads to more frequent disconnection duringwire-drawing. In addition, bainite, which is smaller in work-hardeningamount than pearlite and ineffective in increasing the strength bystrong wire-drawing, should be reduced in the amount. In contrast tothese structures, the amount of the pearlite structure, which iseffective in trapping hydrogen at the interface between cementite andferrite and in reducing the hydrogen accumulated in grain boundary,should be increased for improvement in delayed fracture resistance. Theareal rate of the pearlite structure is recommended to be preferably 90%or more, more preferably 95% or more.

The high-strength bolt steel according to the present invention containsC at 0.5 to 1.0% (mass %, the same shall apply hereinafter), Si at 0.55to 3%, Mn at 0.2 to 2%, P at 0.03% or less (but not 0%), S at 0.03% orless (but not 0%), and Al at 0.3% or less (but not 0%). Hereinafter, thereason for restricting the content of each component will be described.

C is an economical element effective in increasing the strength of bolt,and increase in C content leads to increase in strength. To achieve adesirable strength of bolt, the C content is raised to 0.5% or more,preferably 0.55% or more, and more preferably 0.60% or more. However, anexcessive C content leads to increase in the amount of the proeutectoidcementite precipitated and thus to significant deterioration intoughness and ductility and also in wire-drawing processability. Thus,the C content is 1.0% or less, preferably 0.9% or less, and morepreferably 0.85% or less. When the eutectoid carbon content isrepresented as Ce, the most desirable C content is Ce±0.2% (preferablyCe±0.1% and particularly preferably Ce±0.05%).

Si increases the relaxation resistance of bluing-treated bolt further.It is presumably because Si is solid-solubilized in soft ferrite, themaximum cause of relaxation, and exerts an action to strengthen thesolid solution. Thus, the Si content is 0.55% or more, preferably 0.7%or more, more preferably 1.0% or more, and particularly preferably 1.5%or more. Si has an action to accelerate decarburization of steelmaterial during heating, for example during hot rolling or patenting(e.g., lead patenting). Although the operational condition is normallymodified to prevent decarburization, it is possible to soften thesurface by accelerating decarburization positively and to preventcracking during bolt forging, even when the Si content is increased.However, an excessive Si content leads to deterioration in the ductilityof the core region. Thus, the Si content is 3% or less, preferably 2.5%or less, and more preferably 2.0% or less.

Mn has an action as a deoxidizing agent and also an action to improvethe quenching efficiency of wire rod, and thus to increase theuniformity of the cross sectional structure of the wire rod. The Mncontent is 0.2% or more, preferably 0.4% or more, and more preferably0.5% or more. However, an excessive Mn content leads to generation ofsupercooled structures such as martensite and bainite in the Mnsegregation region and hence to deterioration in wire-drawingprocessability. Thus, the Mn content is 2% or less, preferably 1.5% orless, and more preferably 1.0% or less.

P is an element causing grain boundary segregation and deterioration indelayed fracture resistance. Thus, the P content is reduced to 0.03% orless, preferably 0.02% or less, more preferably 0.015% or less, andparticularly preferably 0.010% or less.

S forms MnS in steel, which becomes the stress-concentration point whena stress is applied. Thus, the S content is preferably as small aspossible, for improvement in delayed fracture resistance. From theviewpoint above, the S amount is limited to 0.03% or less, preferably0.02% or less, more preferably 0.015% or less, and particularlypreferably 0.010% or less.

Al generates nitride and oxide inclusions, lowering wire-drawingefficiency. Thus, the Al content is 0.3% or less, preferably 0.1% orless, and more preferably 0.05% or less; and, in particular when thewire-drawing efficiency is considered more, it is 0.03% or less(preferably 0.02% or less, in particular 0.010% or less). On the otherhand, Al is also effective in improving the delayed fracture resistanceby capturing N in steel to form AlN and reducing the size of crystalgrain, and thus, Al may be added positively. Thus, the Al content is,for example, 0.01% or more, preferably 0.02% or more, and morepreferably 0.03% or more.

The high-strength bolt steel may contain other elements additionally inthe range that does not impair the advantageous effects of the presentinvention, and examples thereof include first supplementary elements(such as Cr and Co), second supplementary elements (such as Ni), thirdsupplementary elements (such as Cu), fourth supplementary elements (suchas Mo, V, Nb, Ti, and W), and fifth supplementary elements (such as B);and these supplementary element may be used alone or in combination oftwo or more as needed. Hereinafter, the supplementary element will bedescribed.

The first supplementary elements, Cr and Co, may be added in the rangesof Cr: 2.5% or less (but not 0%) and Co: 0.5% or less (but not 0%). Crand Co are effective in preventing precipitation of proeutectoidcementite, and thus, particularly useful as the additives for thehigh-strength bolt according to the present invention aimed at reducingthe content of proeutectoid cementite. Such an action is amplified byincrease in the amounts thereof added, and thus, the Cr content isrecommended to be 0.05% or more (for example, 0.1% or more andparticularly preferably 0.2% or more), or the Co content, 0.01% or more(for example, 0.03% or more and particularly preferably 0.05% or more),to make the action more distinctive. An excessive addition leads tosaturation of the action, and is thus uneconomical. Thus, the Cr contentis 2.5% or less (preferably 2.0% or less and more preferably 1.2% orless), and the Co content is 0.5% or less (preferably 0.3% or less andmore preferably 0.2% or less). Only one or both of Cr and Co may beadded.

The second supplementary element Ni may be added in an amount in therange of 1.0% or less (but not 0%). Ni is not effective in improving thestrength of bolt, but effective in increasing the toughness of drawnwire rod. Although the effect is amplified when the Ni content isincreased, the Ni content is recommended to be preferably 0.05% or more,more preferably 0.1% or more, and particularly preferably 0.15% or more,to make the action more distinctive. However, an excessive Ni contentleads to elongation of the period until completion of transformation andthus to expansion of facility and decrease in productivity. Thus, the Nicontent is 1.0% or less, preferably 0.5% or less, and more preferably0.3% or less.

The third supplementary element Cu may be added in an amount in therange of 1.0% or less (but not 0%). Cu is an element effective inimproving the strength of bolt by its precipitation-hardening action.The action is amplified by increase of the Cu content, and, to make theaction more distinctive, the Cu amount is recommended to be preferably0.05% or more, more preferably 0.1% or more, and particularly preferably0.2% or more. However, an excessive Cu content causes embrittlement ofgrain boundary and thus deterioration in delayed fracture resistance.Thus, the Cu amount is 1.0% or less, preferably 0.5% or less, and morepreferably 0.3% or less.

The fourth supplementary elements Mo, V, Nb, Ti, W, and others may beadded in a total amount of 0.5% or less (but not 0%). These elements Mo,V, Nb, Ti, and W form fine carbides/nitrides, which are effective inimproving delayed fracture resistance. The action is amplified byincrease of the total amount of these elements, and the total amount ispreferably 0.02% or more and more preferably 0.05% or more. However, anexcessive total amount of these elements leads to inhibition of delayedfracture resistance and also to deterioration in toughness. Thus, thetotal amount of these elements is 0.5% or less, preferably 0.2% or less,and more preferably 0.15% or less. These elements, Mo, V, Nb, Ti, W, andthe like, may be added alone or in combination of two or more.

The fifth supplementary element B may be added in an amount in the rangeof 0.003% or less (but not 0%). B is added for improvement in quenchingefficiency. The action is amplified by increase of the B content, andthe B content is preferably 0.0005% or more, and more preferably 0.0010%or more, to make the action more distinctive. However, an excessive Bcontent leads to inhibition of toughness. Thus, the B content is 0.003%or less, preferably 0.0025% or less, and more preferably 0.0020% orless.

The elements in balance include Fe and unavoidable impurities.

The bolt steel for use in the present invention preferably has a tensilestrength to a degree allowing the bolt after processing such aswire-drawing or bluing to have a certain strength, and specifically, hasa tensile strength of approximately 1,000 N/mm² or more, preferably1,100 N/mm² or more, more preferably 1,200 N/mm² or more, andparticularly preferably 1,300 N/mm² or more.

The bolt according to the present invention is produced by wire-drawingthe bolt steel (wire rod or steel wire), cold-heading it into bolt, andthen, bluing the bolt in a temperature range of 100 to 500° C. Such abolt has a favorable high tensile strength of 1,200 N/mm² or more(preferably 1,400 N/mm² or more, more preferably 1,500 N/mm² or more,and particularly preferably 1,600 N/mm² or more) and is also superior indelayed fracture resistance and relaxation resistance.

The wire-drawing is performed, because a steel only rolled or forgeddoes not have the dimensional accuracy needed for high-strength bolt andit is difficult to achieve a predetermined final strength. The strongwire drawing results in improvement in fine dispersion of part of thecementite grains in pearlite structure and thus in improvement inhydrogen-trapping capacity, and increases in cracking resistance due tothe alignment of the grains in the wire-drawing direction. Thewire-drawing condition is not particularly limited if it is the strongwire-drawing to a degree giving a particular tensile strength, but, forexample, it is recommended to process the wire rod to a degree that thereduction of area is approximately 30 to 85% (preferably approximately50 to 70%).

The bluing treatment is an essential step in the present invention forutilizing the relaxation resistance endowed by Si. That is, the bluingtreatment is useful because it prevents plastic deformation by the agehardening with C and N and increases the strength and the proof stressratio of bolt; and it is also useful in amplifying the action by the Siaddition above (reinforcement of solid solution in relaxation-causingferrite) and in improving the relaxation resistance markedly, because itprevents thermal settling at a temperature of 100 to 200° C. Because theSi content is increased in the present invention, it is also possible toprevent deterioration in tensile strength and proof stress duringbluing, even when the bluing is performed at high temperature. It isthus possible to improve tensile strength and proof stress andadditionally to raise relaxation resistance. An excessively lower bluingtemperature results in insufficient age hardening and insufficientimprovement in the strength and proof stress ratio of bolt, andconsequently, in insufficient improvement in relaxation resistance.Thus, the bluing temperature is 100° C. or higher, preferably 200° C. orhigher, and more preferably 300° C. or higher. On the other hand, anexcessively higher bluing temperature results in deterioration in thestrength of bolt by softening. Thus, the bluing temperature is 500° C.or lower, preferably 450° C. or lower, and more preferably 400° C. orlower.

EXAMPLES

Hereinafter, the present invention will be described more specificallywith reference to Examples, but it should be understood that the presentinvention is not restricted by the following Examples, and modificationscan be made within the scope of the description above and below, andsuch modification are also included in the technical scope of thepresent invention.

Example 1

Test steels (A to M) respectively having the chemical compositions shownin the following Table 1 were hot-rolled into wires having the wirediameters (8.0 to 11.5 mmφ) shown in the following Table 2, and thewires were patented in the conditions shown in the following Table 2(heating temperature: 940° C., constant temperature transformation: at510 to 620° C. for 4 minutes). The test steel M was converted completelyinto the martensite structure by quenching and tempering for comparison.The structure, degree of decarburization, and tensile strength of thesteel wires obtained were determined. In studying the structure, theproeutectoid ferrite, proeutectoid cementite, bainite and martensite orpearlite structure was separated by the following method; and the arealrate of each structure was determined.

[Separation of Each Structure]

The cross sectional face of a steel wire was embedded, polished andcorroded as it is immersed in alcoholic 5% picrate solution for 15 to 30seconds; and the structure of the D/4 region (D: diameter) was observedunder a scanning electron microscope (SEM) JKA-89CORL manufactured byJapan electron Co., Ltd. Photographs of 5 to 10 visual fields were takenat a magnification of 1,000 to 3,000 times, for determining the pearlitestructure region, and the areal rate of each structure was determined byusing an image-analyzer FRMTOOL-KIT manufactured by Photron Ltd. As forthe bainite and proeutectoid cementite structures, which are less easilydifferentiated from the pearlite structure, the structure shown in FIG.2 (photo replacing drawing) was regarded as the bainite structure, andthe structure shown in FIG. 3 (photo replacing drawing) as theproeutectoid cementite structure. In general tendency of thesestructures, proeutectoid ferrite and proeutectoid cementite depositedalong the original austenite grain boundary, while martensite depositedin the bulky shape. Results are summarized in Table 2. TABLE 1 STEELCHEMICAL COMPOSITION (mass %) ^(※) SYMBOL C Si Mn P S Al N O OTHERS A0.64 1.46 0.66 0.013 0.010 0.002 0.005 0.0014 Cr: 0.68 B 0.64 1.46 0.600.011 0.010 0.002 0.005 0.0019 V: 0.104 C 0.83 0.92 0.74 0.008 0.0060.037 0:006 0.0008 D 0.64 2.05 0.90 0.010 0.008 0.003 0.005 0.0013 Cr:0.98, Ni: 0.26, V: 0.092 E 0.60 1.94 0.93 0.012 0.005 0.033 0.005 0.0014F 0.62 2.54 0.51 0.015 0.012 0.034 0.005 0.0010 Cr: 0.50, Mo: 0.10, Co:0.05 G 0.60 2.89 0.51 0.010 0.008 0.035 0.006 0.0011 Cr: 0.51, B: 0.0015H 0.82 0.55 0.78 0.009 0.007 0.034 0.005 0.0008 Cr: 0.70 I 0.65 1.500.65 0.015 0.014 0.035 0.005 0.0009 Cu: 0.25 J 0.85 0.25 0.77 0.0100.006 0.048 0.004 0.0007 K 0.82 0.26 0.71 0.015 0.009 0.041 0.005 0.0007Cr: 0.18 L 0.77 0.45 0.72 0.012 0.012 0.039 0.005 0.0008 Cr: 0.17 M 0.340.19 0.70 0.016 0.009 0.033 0.003 0.0009 Cr: 0.95, Mo: 0.18^(※)THE ELEMENTS IN BALANCE ARE FE AND UNAVOIDABLE IMPURITIES.

TABLE 2 HOT- PATENTING ROLLED CONSTANT STRUCTURE (AREA %) STEEL WIRETEMPERA- PROEUTEC- PROEUTEC- TENSILE TEST STEEL WIRE DIAMETER TURE TOIDTOID BAI- MARTEN- PEAR- STRENGTH NO. SYMBOL SYMBOL (mm) (° C.) FERRITECEMENTITE NITE SITE LITE (N/mm²) 1 A WA1 10.5 600 10 0 0 0 90 1360 2 WA28.0 600 3 0 0 0 97 1358 3 WA3 8.0 620 5 0 0 0 95 1287 4 B WB1 10.5 60010 0 0 0 90 1370 5 WB2 8.0 600 5 0 0 0 95 1346 6 WB3 8.0 620 7 0 0 0 931305 7 C WC 10.5 560 5 5 0 0 90 1314 8 D WD 10.5 600 10 0 0 0 90 1450 9E WE 11.5 560 15 0 0 0 85 1220 10 F WF 10.5 600 10 0 0 0 90 1381 11 G WG10.5 600 15 0 0 0 85 1398 12 H WH 10.5 560 5 5 0 0 90 1305 13 I WI 8.0600 5 0 0 0 95 1399 14 J WJ 8.0 510 5 0 0 0 95 1268 15 K WK 8.0 525 5 00 0 95 1305 16 L WL 8.0 535 10 0 0 0 90 1310 17 M WM 11.0 QUENCHING(880°C. × 30 min.→OQ). TEMPERING(460° C. × 90 min.→WC), 100% MARTENSITESTRUCTUREOQ: OIL QUENCHING, WC: WATER COOLING

The respective steel wires were wire-drawn tightly to the wire diameters(7.06 mmφ or 5.25 mmφ) shown in the following Table 3 (reduction ofarea: 55 to 62%); the wire-drawing efficiency was evaluated; and thetensile strength of the strongly drawn wirerods obtained and the delayedfracture resistance were determined. The criteria for evaluation ofwire-drawing efficiency are as follows: The wire-drawing efficiency andthe delayed fracture resistance were evaluated by the following methods:

[Wire-Drawing Efficiency]

Favorable: Drawn to a predetermined wire diameter without problem, andno abnormal breakage observed in the tensile test after wire-drawing.

Unfavorable: Abnormal breakage such as cuppy breakage and longitudinalcrack observed in the tensile test during or after wire-drawing

[Delayed Fracture Resistance]

From the strongly drawn wire rods, stud bolts of M8×P1.25 shown in FIG.1 [FIG. 1A, from steel wire having a wire diameter of 7.06 mmφ)] andstud bolts of M6×P1.0 [FIG. 1B, from the steel wire having a wirediameter of 5.25 mmφ] were prepared, and the delayed fracture test wasperformed. In the delayed fracture test, a bolt was immersed in acid(15% HCl×30minute), washed with water, dried, and then, left in airunder stress (loaded stress: 90% of tensile strength); and presence orabsence of fracture evaluated after 100 hours (∘: no fracture, x:fracture). TABLE 3 DRAWN WIRE ROD STEEL STRONG WIRE-DRAWING STEP TENSILEDELAYED TEST STEEL WAVE REDUCTION POST-WIRE-DRAWING WIRE-DRAWINGSTRENGTH FRACTURE NO. SYMBOL SYMBOL OF AREA (%) WIRE DIAMETER (mm)EFFICIENCY (N/mm²) RESISTANCE 1 A WA1 55 7.06 FAVORABLE 1609 ∘ 2 WA2 575.25 FAVORABLE 1615 ∘ 3 WA3 57 5.25 FAVORABLE 1561 ∘ 4 B WB1 55 7.06FAVORABLE 1619 ∘ 5 WB2 57 5.25 FAVORABLE 1623 ∘ 6 WB3 57 5.25 FAVORABLE1584 ∘ 7 C WC 55 7.06 FAVORABLE 1637 ∘ 8 D WD 55 7.06 FAVORABLE 1699 ∘ 9E WE 62 7.06 FAVORABLE 1507 ∘ 10 F WF 55 7.06 FAVORABLE 1622 ∘ 11 G WG55 7.06 FAVORABLE 1632 ∘ 12 H WH 55 7.06 FAVORABLE 1624 ∘ 13 I WI 575.25 FAVORABLE 1658 ∘ 14 J WJ 57 5.25 FAVORABLE 1640 ∘ 15 K WK 57 5.25FAVORABLE 1631 ∘ 16 L WL 57 5.25 FAVORABLE 1628 ∘ 17 M WM 59 7.06 — 1318x

Each of the strongly drawn wire rods thus obtained was subjected to abluing treatment at a temperature of 200 to 400° C. The tensile strengthand the 0.2% proof stress of the bluing and non-bluing treated wire rodswere determined. Separately, a relaxation test was performed accordingto JIS G3538. The temperature of the relaxation test was 150° C. In thepresent test, a test piece was held at a suitable distance, and a load(applied load) W1 equivalent to 80% of the load of the 0.2% proof stresswas applied; the applied load was gradually lowered, while a movableweight (weight for adjusting elongation of the test piece) was adjustedto make the grasp length of the test piece (GL: 300 mm) constant; theload W2 after 10 hours was determined; and the relaxation value wascalculated according to the following Formula:Relaxation value (%)=[(W1·W2)/W1]×100

In addition, each stud bolt prepared above after bluing was alsoevaluated in a delayed fracture test similar to that above. Results aresummarized in Table 4 and FIG. 4. TABLE 4 0.2% STEEL BLUING TENSILEPROOF APPLIED RELAXATION DELAYED TEST STEEL WIRE TEMPERATURE STRENGTHSTRESS LOAD VALUE FRACTURE NO. SYMBOL SYMBOL (° C.) (N/mm²) (N/mm²)(N/mm²) (%) RESISTANCE 2 A WA2 — 1615 1436 1149 10.55 —  2A 200 17131642 1314 9.14 ∘  2B 300 1700 1638 1310 8.87 ∘  2C 400 1681 1628 13028.39 ∘ 3 WA3 — 1561 1363 1090 10.48 —  3A 200 1671 1595 1276 7.72 ∘ 5 BWB2 — 1623 1453 1162 10.50 —  5A 200 1746 1709 1367 8.76 ∘ 6 WB3 — 15841389 1111 10.30 —  6A 200 1690 1617 1294 8.87 ∘  7A C WC 200 1754 17051364 9.11 ∘  7B 300 1734 1643 1314 8.89 ∘  7C 400 1728 1624 1299 8.47 ∘ 8A D WD 200 1821 1765 1412 7.81 ∘  8B 300 1807 1715 1372 7.55 ∘  8C 4001787 1699 1359 7.41 ∘  9A E WE 200 1628 1594 1275 8.21 ∘  9B 300 16111542 1234 8.10 ∘  9C 400 1608 1538 1230 7.99 ∘ 10A F WF 200 1741 17011361 7.51 ∘ 10B 300 1738 1698 1358 7.40 ∘ 11A G WG 200 1758 1724 13797.31 ∘ 11B 300 1751 1715 1372 7.26 ∘ 12A H WH 200 1711 1668 1334 9.55 ∘12B 300 1698 1625 1300 9.26 ∘ 13A I WI 200 1751 1711 1369 8.67 ∘ 13B 3001742 1708 1366 8.11 ∘ 14A J WJ 200 1758 1680 1344 12.31 ∘ 14B 300 17501658 1326 10.49 ∘ 15A K WK 200 1740 1660 1328 12.52 ∘ 15B 300 1727 15511241 10.70 ∘ 16A L WL 200 1745 1671 1337 11.01 ∘ 16B 300 1734 1572 125810.51 ∘

The quenched and tempered steel (martensite steel) M was insufficient indelayed fracture resistance (see Table 3).

The pearlite steels J to L were improved in delayed fracture resistance(see Tables 3 and 4). However, these steels having a Si content of lessthan 0.55% were limited in the relaxation resistance (see Table 4).

In contrast, both bluing and unbluing-treated steels A to I, which areall pearlite steels, were superior in delayed fracture resistance (seeTables 3 and 4). In addition, the bluing-treated steels having a Sicontent of 0.55% or more were more improved in relaxation resistance(white and black circles in FIG. 4). As apparent from FIG. 4, it is notpossible to improve the relaxation resistance without bluing treatmenteven if the Si content is 0.55% or more (cross in FIG. 4), but possibleto improve the relaxation resistance by combination of increase in theSi content with bluing treatment.

In summarizing the invention above, the high-strength bolt according tothe invention is characterized by having a relaxation resistancesignificantly increased by addition of Si in an amount of more than aparticular value.

The high-strength bolt according to the invention has a tensile strengthof 1,200 N/mm² or more that is superior in delayed fracture resistanceand relaxation resistance, characterized by being prepared by:wire-drawing a bolt steel containing the following elements: C: 0.5 to1.0% (mass %, the same shall apply hereinafter), Si: 0.55 to 3%, Mn: 0.2to 2%, P: 0.03% or less (but not 0%), S: 0.03% or less (but not 0%), andAl: 0.3% or less (but not 0%), having at a total areal rate ofproeutectoid ferrite, proeutectoid cementite, bainite and martensite ofless than 20%, and containing pearlite in balance; cold-heading the wireinto a bolt shape; and then bluing the bolt in a temperature range of100 to 500° C.

The high-strength bolt according to the present invention may containadditionally Cr at 2.5% or less (but not 0%), Co at 0.5% or less (butnot 0%), Ni at 1.0% or less (but not 0%), Cu at 1.0% or less (but not0%), B at 0.003% or less (but not 0%), and others, and may contain Mo,V, Nb, Ti, W, and others in a total amount in the range of 0.5% or less(but not 0%).

The high-strength bolt according to the present invention preferablycontains additionally at least one element selected from Cr at 2.5% orless (but not 0%) and Co at 0.5% or less (but not 0%). It morepreferably contains additionally Ni at 1.0% or less (but not 0%). Itmore preferably contains additionally at least one element selected fromMo, V, Nb, Ti, and W in a total amount in the range of 0.5% or less (butnot 0%). It more preferably contains B at 0.003% or less (but not 0%).

The high-strength bolt according to the present invention morepreferably contains additionally Ni at 1.0% or less (but not 0%).

The high-strength bolt according to the present invention morepreferably contains Cu at 1.0% or less (but not 0%).

The high-strength bolt according to the present invention morepreferably contains additionally at least one element selected from Mo,V, Nb, Ti, and W in a total amount in the range of 0.5% or less (but not0%).

The high-strength bolt according to the present invention preferablycontains B at 0.003% or less (but not 0%).

The balance of the high-strength bolt according to the present inventionmay be Fe and inevitable impurities.

The present invention overcomes various problems associated with boltsprepared from pearlite steel materials superior in delayed fractureresistance. That is, it is possible to improve the relaxation resistanceof bolt drastically by adding Si in a certain amount or more.

This application is based on Japanese Patent Application No. 2004-057379filed on Mar. 2, 2004, the contents of which are hereby incorporated byreference.

INDUSTRIAL APPLICABILITY

By the present invention in the configuration above, it was possible toproduce a high-strength bolt having a tensile strength of 1,200 N/mm² ormore that is superior in delayed fracture resistance and relaxationresistance, by adding Si in a certain amount or more.

1. A high-strength bolt having a tensile strength of 1,200 N/mm² or morethat is superior in delayed fracture resistance and relaxationresistance, wherein the bolt is prepared by: wire-drawing a bolt steelcontaining the following elements: C: 0.5 to 1.0% (mass %, the sameshall apply hereinafter), Si: 0.55 to 3%, Mn: 0.2 to 2%, P: 0.03% orless (but not 0%), S: 0.03% or less (but not 0%), and Al: 0.3% or less(but not 0%), and containing proeutectoid ferrite, proeutectoidcementite, bainite and martensite at a total areal rate of less than 20%and pearlite in balance; cold-heading the wire into a bolt shape; andthen bluing the bolt in a temperature range of 100 to 500° C.
 2. Thehigh-strength bolt according to claim 1, wherein the bolt steel furthercomprises at least one of the following elements: Cr: 2.5% or less (butnot 0%) and Co: 0.5% or less (but not 0%).
 3. The high-strength boltaccording to claim 1, wherein the bolt steel further comprises Ni at1.0% or less (but not 0%).
 4. The high-strength bolt according to claim1, wherein the bolt steel further comprises Cu at 1.0% or less (but not0%).
 5. The high-strength bolt according to claim 1, wherein the boltsteel further comprises at least one element selected from Mo, V, Nb,Ti, and W in a total amount of 0.5% or less (but not 0%).
 6. Thehigh-strength bolt according to claim 1, wherein the bolt steel furthercomprises B at 0.003% or less (but not 0%).
 7. The high-strength boltaccording to claim 2, wherein the bolt steel further comprises Ni at1.0% or less (but not 0%).
 8. The high-strength bolt according to claim2, wherein the bolt steel further comprises at least one elementselected from Mo, V, Nb, Ti, and W in a total amount of 0.5% or less(but not 0%).
 9. The high-strength bolt according to claim 2, whereinthe bolt steel further comprises B at 0.003% or less (but not 0%). 10.The high-strength bolt according to claim 7, wherein the bolt steelfurther comprises at least one element selected film Mo, V, Nb, Ti, andW in a total amount of 0.5% or less (but not 0%).
 11. The high-strengthbolt according to claim 1, wherein the elements in balance are Fe andinevitable impurities.